![]() ![]() To understand the double slit interference pattern, we consider how two waves travel from the slits to the screen, as illustrated in Figure 27.13. (c) When light that has passed through double slits falls on a screen, we see a pattern such as this. Wave action is greatest in regions of constructive interference and least in regions of destructive interference. (b) Double slit interference pattern for water waves are nearly identical to that for light. We can only see this if the light falls onto a screen and is scattered into our eyes. These waves overlap and interfere constructively (bright lines) and destructively (dark regions). (a) Light spreads out (diffracts) from each slit, because the slits are narrow. Figure 27.11 shows the pure constructive and destructive interference of two waves having the same wavelength and amplitude.įigure 27.12 Double slits produce two coherent sources of waves that interfere. We illustrate the double slit experiment with monochromatic (single λ λ) light to clarify the effect. Young used sunlight, where each wavelength forms its own pattern, making the effect more difficult to see. Why did Young then pass the light through a double slit? The answer to this question is that two slits provide two coherent light sources that then interfere constructively or destructively. Incoherent means the waves have random phase relationships. By coherent, we mean waves are in phase or have a definite phase relationship. Furthermore, Young first passed light from a single source (the Sun) through a single slit to make the light somewhat coherent. Why do we not ordinarily observe wave behavior for light, such as observed in Young’s double slit experiment? First, light must interact with something small, such as the closely spaced slits used by Young, to show pronounced wave effects. ![]() Without diffraction and interference, the light would simply make two lines on the screen. Here pure-wavelength light sent through a pair of vertical slits is diffracted into a pattern on the screen of numerous vertical lines spread out horizontally. Figure 27.10 Young’s double slit experiment. ![]()
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